1. Field of the Invention
This invention relates to a power supply circuit providing an integrated circuit with a supply voltage, specifically to a power supply circuit capable of supplying a constant voltage insensitive to change in ambient temperature.
2. Description of the Related Art
Generally speaking, it is not easy to obtain a constant voltage with an integrated circuit, because characteristics of transistors and resistances in the integrated circuit are affected by change in ambient temperature. Hence, an element insensitive to the temperature change such as a zener diode is often used to obtain the constant voltage. A large current, e.g. 100 mA to 200 mA, flows through a power supply transistor, which supplies the current to a power supply line in the integrated circuit. Because it is difficult for the integrated circuit to include the power supply transistor with such a larger current capacity, the power supply transistor is placed outside the integrated circuit.
FIG. 2 shows an example of a power supply circuit according to a conventional art. A power supply voltage of 11V, for example, is provided to a terminal 2 located outside the integrated circuit, from which the power supply voltage is applied to the inside of the integrated circuit 1 through a resistance 3 and a pin 4 of the integrated circuit 1. The voltage at the pin 4 is applied to a zener diode 5, which generates a constant voltage between both ends of it, insensitive to the change in the ambient temperature. The constant voltage across the zener diode 5 is applied to a bleeder resistance 8 consisting of a resistance 6 and a resistance 7. The bleeder resistance 8 divides the constant voltage proportionally to a ratio between the resistance 6 and the resistance 7. The divided voltage is led to outside of the integrated circuit 1 through a buffer circuit 9 and a pin 10 of the integrated circuit 1.
A power supply transistor 11 is made of a discrete transistor disposed outside the integrated circuit 1, and provides a constant voltage from its emitter in accordance with a voltage applied to its base. The emitter voltage of the power supply transistor 11 is fed back to the inside of the integrated circuit 1 through a pin 12 as a power supply voltage of the integrated circuit 1.
The pin 12 makes a so-called power supply pin of the integrated circuit 1, from which the power supply voltage is provided to various circuit blocks 13, 14 and 15 in the integrated circuit 1. Thus the constant voltage can be supplied to the circuit blocks in the integrated circuit, according to the power supply circuit of FIG. 2.
The configuration shown in FIG. 2 can provide the pin 10 with the voltage insensitive to the change in the ambient temperature. However, the power supply transistor 11 has temperature characteristics specific to a discrete device, and a voltage between its base and emitter fluctuates, albeit only slightly. As a result, the power supply voltage provided to the circuit blocks 13, 14 and 15 fluctuates.
It is conceivable to apply a temperature dependent voltage to the pin 10, so that the fluctuation in the applied voltage would cancel the fluctuation due to the temperature characteristics of the power supply transistor 11. However, it is difficult to implement.
As described above, eliminating the fluctuation in the power supply voltage has been difficult.